In general, when polymerizable resin bases (e.g. monomers or monomer mixtures) are polymerized, shrinkage occurs. As it has been pointed out in, e.g., “Ring-Opening Polymerization with Expansion in Volume” by William J. Bailey et al, ACS Symposium 59, No. 4; pages 38-59 (1977), most of the critical shrinkage occurs after the gel point in cross-linked materials, or when the monomer-polymer mixture approaches the glass transition point in linear thermoplastic materials. This publication also notes that it is desirable for many applications in polymer technology that polymerization should be accompanied by near zero shrinkage or even by expansion. Examples of areas where near zero shrinkage is desirable are: strain-free composites, potting resins, high gloss coatings, binders for solid propellants, impression materials and structural adhesives. Zero shrinkage materials find particular application in R.I.M. (reaction-in-molding) technology.
However, even expansion may be desirable in such areas as e.g. precision castings, high strength adhesives, pre-stressed plastics, rock-cracking materials, elastomeric sealants and dental fillings.
Dental plastic fillings are based on the principle of polymerization of resin bases including monomers or oligomers. This can give rise to shrinkage when the plastic dental filling material is polymerised. This means that a small micro-crack is opened between the tooth and the filling. The crack can cause secondary caries or discoloration of the plastic filling. Micro-cracks give rise to a degradation of the mechanical properties of the composite material. In the field of bone cement the shrinkage creates porous structures between the bone cement and implant cement. This can also give rise to degradation of the mechanical properties and failure of implants. In the field of impression materials, shrinkage can cause dimension problems which can lead to misfit.
Thus, it would clearly be useful to utilize filler materials that could counteract the shrinkage normally occurring upon curing of polymerizable resin bases, and which generally may be used in polymerization processes (i.e. not restricted for practical purposes to thermal cure).
Zirconia has widespread utility as a filler ingredient in composite materials, e.g. dental materials. Zirconia can exist in three principal crystalline phases: the tetragonal phase, the cubic phase and the monoclinic phase. The specific volume (density−1) of the three phases is 0.16, 0.16 and 0.17 cm3/g, respectively.